Studies were undertaken to determine the prevalence and natural behavior of type C RNA viruses of cat, chicken, mouse, and man. Studies of spontaneous expression of endogenous type C RNA viruses in lymphocyte cultures derived from mice of different strains and humans showed that a spontaneous and predictable release of B-tropic virus occurred in lymphocytes derived from three BALB/c mice of certain age groups. Virus release was not observed in three lymphocyte cultures derived from other mouse strains or humans. The group specific antigen of avian reticuloendotheliosis virus was isolated and characterized. A sensitive radioimmunoassay was developed for the detection of REV antigens and antibodies. The technique of radioimmunoassay was simplified by the use of killed Staphylococci as immunoabsorbant for precipitating antigen-antibody complexes. Feline cells were found to be consistently more sensitive than rat cells to the cell transforming effects of primate sarcoma viruses including the cell-transforming HL23 virus. Repeated tests failed to provide evidence to suggest that human sera of cancer patients or laboratory workers contain viral neutralizing antibodies against the primate sarcoma viruses. BIBLIOGRAPHIC REFERENCES: Premkumar-Reddy, E., Price, P.J., Chung, K.C. and Sarma, P.S.: Continuous culturing of murine splenic B-lymphocytes: Synthesis and surface deposition of IgM and putative IgD molecules. Cell 8: 397-403, 1976. Devare, S.G., Stephenson, J.R., Sarma, P.S. and Aaronson, S.A.: Bovine lymphosarcoma: Development of a radioimmunologic technique for detection of the etiologic agent. Science 194: 1428-1430, 1976.